1. Field of the Invention
This invention relates generally to DC motors and, more particularly, is directed to an improved circuit for driving a DC motor.
2. Description of the Prior Art
Various types of brushless DC motors are known, such motors generally including at least one pair of magnetic poles, usually formed of permanent magnet north and south pole pieces, secured to either the rotor (brushless type) or the stator (commutator type) and corresponding energizable coils mounted on the other of the stator (brushless type) or the rotor (commutator type) with each coil having conductor segments for carrying current in directions which are normal to the magnetic flux generated by the permanent magnet pole pieces. With such an arrangement, the rotor is caused to rotate relative to the stator by a torque produced by the interaction of magnetic flux generated by the pole pieces and the current flow through the coils with the direction of rotation being perpendicular both to the direction of magnetic flux and the direction of current flow.
Known brushless DC motors of the above type are shown, for example, in U.S. Pat. Nos. 3,383,574 and 3,517,289. In such DC motors, two coil windings are displaced from one another by an electrical angle of 90.degree.. In order to provide energization of the coils, two position sensing devices, such as two Hall-effect elements, detect the rotational position of the rotor with respect to the stator and produce sinusoidal drive signals which are used to alternately energize the windings for rotating the rotor with a substantially constant torque regardless of the rotational position of the rotor. In particular, one Hall-effect element produces a drive current signal proportional to sin .theta. for energizing one of the windings and the other Hall-effect element produces a drive current signal proportional to cos .theta. for energizing the other winding.
However, because of such arrangement, positive and negative polarity power sources are required for operating the DC motor. Also, since two Hall-effect elements are used, there may be fluctuation in the DC level between the two drive current signals, resulting in a torque having undesirable ripple. In addition, there may be a problem with electrical noise in such DC motors, particularly during switching in DC motors of the commutator type.